The official CM 13 Nightlies are now live for all the Snapdragon 801-powered Galaxy S5 phones. If you own one of these devices, then you should use the tutorial listed in this post in order to learn how to successfully flash a brand new custom Marshmallow ROM.

The CM 13 is based on the AOSP Android 6.0 builds and you can find multiple extra things added by the CyanogenMod team. Before starting the installation process of this new ROM you need to read the following guidelines:

use this post only for the Samsung Galaxy S5 LTE models.
- don't try to update the Verizon nor AT&T models with the files that you will find in this post.

your phone has to be rooted and it should be running a custom Recovery that's based on either CWM or TWRP for the installation to be completed successfully.
- the installed custom Recovery has to be upgraded to its latest version, so that it will support the newest Android 6.0-based custom ROMs.

this is a tutorial that needs to be applied by advanced users, we cannot be blamed in case you end up bricking your phone nor in case you lose any important data in the process.
- make sure to backup all important files before starting the CM 13 installation.

in the tutorial you will find two extra preparations steps: the NANDROID Backup and the Factory Reset ones.
- do not skip those steps.
- the NANDROID Backup creates a full restore point for your smartphone, one that you can easily use in case the new ROM doesn't boot in normal mode.
- the Factory Reset process wipes clean the system memory partition, and it removes the old ROM completely from your device.
- by factory resetting your phone you will make sure that the new CM 13 ROM will boot in normal mode.

if you need to save data from installed applications, then you should use Helium for Android.

make sure to verify that battery charge level of your phone, there should be more than 50% charge left before it drains completely.

read the whole guide and make sure not to skip any steps in case you're looking to flash the custom ROM at your first attempt.

After you install the new CM 13 build you should go to CyanogenMod Updates menu under Settings> About Phone, in order to check for new Nightly builds. Make sure to update the installed CM 13 ROM on a weekly basis, so that you will get a much more stable and better Marshmallow experience each time.

After the ROM is done installing you should flash the Gapps zip, too.
- do it in the same way you did with the CM 13 file.

When all the new files are done installing you can reboot the phone in normal mode.
- the first boot process of this new ROM is going to take a while before it loads all the new files.

Use the default Gmail account in order to regain control over your phone once again.

These were all the steps that you had to take in order to complete the CM 13 Nightly Android 6.0 installation on your Galaxy S5 phone.

If the first boot process of this new ROM is going to take more than 20 minutes or the device enters in a boot loop, then you will have to try and repeat the installation. If you retry the CM 13 Android 6.0 flashing process then you should make sure not to skip any steps and to verify that your phone meets all requirements.